A rolling-bearing unit is used to support the wheels of an automobile such that they rotate freely with respect to the suspension. Also, in order to control an anti-lock brake system (ABS) or traction-control system (TCS), it is necessary to detect the rotation speed of the wheels. In order to do this, it is common practice to support the wheels such that they rotate freely with respect to the suspension and to detect the rotation speed of the wheels by using a wheel-rotation support apparatus with encoder in which a rotational speed detector is assembled in the rolling-bearing unit.
FIG. 9 and FIG. 10 show one example of prior art construction of a wheel-rotation support apparatus with encoder that is used for the purpose mentioned above as disclosed in Japanese Patent Publication Jitsukai Hei 7-31539. This wheel-rotation support apparatus with encoder supports a hub 2, which is a rotating race that rotates during use, on the radial inside of an outer race 1, which is a stationary race that does not rotate even during use. A sensor 4 that is supported by the outer race 1 detects the rpm of an encoder 3 that is fastened to part of the hub 2. In order to do this, a plurality of rows of outer-raceways 5, which are stationary raceways, are formed around the inner peripheral surface of the outer race 1. Moreover, inner-raceways 8, which are rotating raceways, are formed around the outer peripheral surface of the hub 2 and around the outer peripheral surface of the inner race 7. The inner race 7 fits around the hub 2 and is fastened to the hub 2 by a nut 6, so as to form the rotating races together with the hub 2. In addition, there is a plurality of rolling elements 9 located between the each of the inner-raceways 8 and outer-raceways 5 such that they are rotatably held by retainers 10, and that they rotatably support the hub 2 and inner race 7 on the inside of the outer race 1.
Also, a flange 11 for installing the wheel and a rotating member for braking such as a disk rotor, or drum is formed on the outside end in the axial direction of the hub 2 (the outside with respect to the axial direction is the side toward the outside in the width direction when the apparatus is assembled in the automobile, or the left side in all of the attached drawings except for FIGS. 2 to 4 and FIG. 10) in the section that protrudes outward in the axial direction from the outside end of the outer race 1. Moreover, an installation section 12 for installing the outer race 1 to the suspension is formed on the inside end of the outer race 1 (the inside with respect to the axial direction is the side toward the center in the width direction when the apparatus is assembled in the automobile, and is the right side in all of the drawings except for FIGS. 2 to 4 and FIG. 10). Also, the clearance between the opening on the outside end of the outer race 1 and the outer peripheral surface around the middle section of the hub 2 is sealed by a seal ring 13. In the case of a rolling-bearing unit for a heavy vehicle, tapered rollers may be used instead of the balls shown in the figures as the plurality of rolling elements 9.
In order to assemble a rotation speed detector in this kind of rolling-bearing unit, an encoder 3 is fastened around the outer peripheral surface on the inside end of the inner race 7 in the section that is separated from the inner-ring raceway 8. This encoder 3 is formed from a magnetic metal plate such as carbon steel plate by plastic working into a generally circular shape having an L-shaped cross section, and is such that it comprises a cylindrical section 14 and circular-ring section 15, and of these, the cylindrical section 14 is fastened around the inside end of the inner race 7 by interference-fitting. Also, by forming a plurality of through holes 16 in the circular-ring section 15 that have a long slit shape in the radial direction of the circular-ring section 15 and such that they are uniformly spaced in the circumferential direction; the magnetic characteristics of the circular-ring section 15 alternately change at equal intervals in the circumferential direction.
Furthermore, a cover 17 is attached to the opening on the inside end of the outer race 1 such that it faces the inner surface of the circular-ring section 15 of the encoder 3, and this cover 17 covers the opening on the inside end of the outer race 1. The sensor 4 is fastened to this cover with a specified location relationship. When this sensor 4 is fastened to the cover 17, the detection section of the sensor 4 faces through a small clearance the inside surface of the circular-ring section 15 of the encoder 3.
When using this wheel-rotation support apparatus with encoder, the installation section 12 that is formed around the outer peripheral surface of the outer race 1 is fastened to the suspension by a bolt (not shown in the figure), and the wheel is fastened to the flange 11 that is formed around the outer peripheral surface of the hub 2 by the studs 18 that are formed in the flange 11 such that the wheel is supported so that it can rotate freely with respect to the suspension, When the wheel rotates in this state, the through holes 16 that are formed in the circular-ring section 15 and the column sections that are located between pairs of adjacent through holes 16 in the circumferential direction alternately pass nearby the detection section of the sensor 4. As a result, the density of the magnetic flux flowing though the sensor 4 changes, and thus the output of the sensor 4 changes. The frequency at which the output of the sensor 4 changes in this way is proportional to the rpm of the wheel. Therefore, by sending the output from the sensor 4 to a controller (not shown in the figure), it is possible to sufficiently perform ABS or TCS control.
With the prior art construction shown in FIG. 9 and FIG. 10, an encoder made of magnetic metal plate was used as the encoder 3, and the permanent magnet for generating the magnetic flux was located on the side of the sensor 4. On the other hand, various kinds of rotation speed detectors have been proposed in which an encoder that rotates with the rotating race is made of permanent magnet. In the case of an encoder made of a permanent magnet, S-poles and N-poles are arranged around the detected surface that faces the detection section of the sensor such that they alternate at uniform intervals. When using this kind of encoder made of a permanent magnet, the construction of the sensor becomes simplified, and the rpm can be detected with good accuracy even at low running speeds, much research for putting it into practice has been performed in this area in recent years.
In the case of an encoder made of a permanent magnet, there is a possibility that the encoder itself will absorb magnetic powder, so the installation conditions were limited. For example, in the case of the prior art construction shown in FIG. 9, the encoder 3 was located in a section that was completely blocked from the external space by the seal ring 13 and cover 17, so even in the case of using an encoder made of a permanent magnet in the place of this encoder 3 there is no particular problem. In other words, in the case of the construction shown in FIG. 9, foreign matter that is floating in the external space cannot enter into the area where the encoder 3 is located, so even when an encoder made of a permanent magnet is used in the place of this encoder 3, magnetic powder is not absorbed by the permanent magnet encoder.
On the other hand, in the case of an encoder that is assembled in the rotation support apparatus for a non-independent suspension such as for a driven wheel or truck, this encoder may have to be located further in the external space than the seal that partitions off the external space and the space inside the bearing where the rolling elements are located. In this case, magnetic power will adhere to the encoder unless some kind of countermeasures are taken, and there is a possibility that reliability of the rpm detection by the sensor will be lost. By simply separating the section where the encoder is located from the external space by a seal member with very a very good seal in order to prevent magnetic powder from adhering to the encoder in this way, the torque required to turn the wheel becomes larger, and causes operating performance of the automobile to become worse.